docs: add media-query-observer.md

docs/api/ui/media-query-observer.md

+MediaQueryObserver 对象，用于监听页面 media query 状态的变化，如界面的长宽是不是在某个指定的范围内。
+
+### uni.createMediaQueryObserver([this])
+创建并返回一个 ``MediaQueryObserver`` 对象实例。
+
+**this说明：**
+
+自定义组件实例。**支付宝小程序不支持此参数，传入仅为抹平写法差异**
+
+### MediaQueryObserver 对象的方法列表
+
+|方法|说明|
+|:-|:-|
+|MediaQueryObserver.observe(Object descriptor, function callback)|开始监听页面 media query 变化情况|
+|MediaQueryObserver.disconnect()|停止监听，回调函数将不再触发|
+
+**observe 回调函数包含一个参数**
+
+||类型|说明|
+|:-|:-|:-|
+|intersectionRatio|boolean|页面的当前状态是否满足所指定的 media query|
+
+### 代码示例
+
+```
+<template>
+    <view class="content">
+        <view class="">
+            matches: {{matches}}
+        </view>
+        <view>
+            landscape: {{landscape}}
+        </view>
+            <button type="success" @click="remove">destroy</button>
+        </view>
+    </view>
+</template>
+
+<script>
+    let landscapeObs, interObs
+    export default {
+        data() {
+            return {
+                matches: false,
+                landscape: false,
+            }
+        },
+        onLoad() {
+
+        },
+        mounted() {
+            this.testMediaQueryObserver()
+            this.landscapeObserver()
+
+        },
+
+        methods: {
+            changeMinWidth() {
+                console.log("======= changeMinWidth ======")
+                var that = this
+                setTimeout(() => {
+                    that.minWidth =  420
+                    console.log("that.minwidth", that.minWidth)
+                }, 1000)
+
+            },
+            testMediaQueryObserver() {
+                this.mediaQueryOb = uni.createMediaQueryObserver(this)
+
+                this.mediaQueryOb.observe({
+                    minWidth: 305,
+                    maxWidth: 400,
+                    orientation: 'portrait'  
+                }, matches => {
+                    console.log('######### matches #########', matches)
+                    this.matches = matches;
+                })
+                console.log('########## create mediaQueryOb ###########', this.mediaQueryOb)
+            },
+            landscapeObserver() {
+                // console.log('--------- this----------', this)
+                landscapeObs = uni.createMediaQueryObserver(this)
+                landscapeObs.observe({
+                    }, matches => {
+                        this.landscape = matches
+                })
+                console.log('--------- landscapeObs -------', landscapeObs)
+                return landscapeObs
+            },
+            remove() {
+                this.mediaQueryOb.disconnect()
+                landscapeObs.disconnect()
+                console.log('########## destroy mediaQueryOb ###########', this.mediaQueryOb)
+            }
+        }
+    }
+</script>
+
+<style>
+    .content {
+        text-align: center;
+        height: 400upx;
+    }
+</style>
+```